153.2.2 Upper Ocean Temperature
Depth-averaged 0 to 700 m ocean temperature trends from 1971 to 2010 are positive over most of the globe (Levitus et al., 2009; Figure 3.1a). The warming is more prominent in the Northern Hemisphere (NH), especially the North Atlantic. This result holds in different analyses, using different time periods, bias corrections and data sources (e.g., with or without XBT or MBT data) (e.g., Palmer et al., 2007; Durack and Wijffels, 2010; Gleckler et al., 2012; Figures 3.1 and 3.9). However, the greater volume of the SH oceans increases the contribution of their warming to global heat content. Zonally averaged upper ocean temperature trends show warming at nearly all latitudes and depths (Levitus et al., 2009, Figure 3.1b). A maximum in warming south of 30°S appears in Figure 3.1b, but is not as strong as in other analyses (e.g., Gille, 2008), likely because the data are relatively sparse in this location so anomalies are attenuated by the objectively analyzed fields used for Figure 3.1 and because warming in the upper 1000 m of the Southern Ocean was stronger between the 1930s and the 1970s than between the 1970s and 1990s (Gille, 2008). Another warming maximum is present at 25°N to 65°N. Both warming signals extend to 700 m (Levitus et al., 2009, Figure 3.1b), and are consistent with poleward displacement of the mean temperature field. Other zonally averaged temperature changes are also consistent with poleward displacement of the mean temperatures. For example, cooling at depth between 30°S and the equator (Figure 3.1b) is consistent with a southward shift of cooler water near the equator. Poleward displacements of some subtropical and subpolar zonal currents and associated wind changes are discussed in Section 3.6. Globally averaged ocean temperature anomalies as a function of depth and time (Figure 3.1c) relative to a 1971–2010 mean reveal warming at all depths in the upper 700 m over the relatively well-sampled 40-year period considered. Strongest warming is found closest to the sea surface, and the near-surface trends are consistent with independently measured SST (Chapter 2). The global average warming over this period is 0.11 to 0.13 °C per decade in the upper 75 m, decreasing to 0.015°C per decade by 700 m (Figure 3.1c). Comparison of Argo data to Challenger expedition data from the 1870s suggests that warming started earlier than 1971, and was also larger in the Atlantic than in the Pacific over that longer time interval (Roemmich et al., 2012). An observational analysis of temperature in the upper 400 m of the global ocean starting in the year 1900 (Gouretski et al., 2012) finds warming between about 1900 and 1945, as well as after 1970, with some evidence of slight cooling between 1945 and 1970. The globally averaged temperature difference between the ocean surface and 200 m (Figure 3.1d) increased by about 0.25ºC from 1971 to 2010 (Levitus et al., 2009). This change, which corresponds to a 4% increase in density stratification, is widespread in all the oceans north of about 40°S. A potentially important impact of ocean warming is the effect on sea ice, floating glacial ice and ice sheet dynamics (see Chapter 4 for a discussion of these topics). Although some of the global integrals of UOHC neglect changes poleward of ±60° (Ishii and Kimoto, 2009) or ±65° (Domingues et al., 2008) latitude, at least some parts of the Arctic have warmed: In the Arctic Ocean, subsurface pulses of relatively warm water of Atlantic origin can be traced around the Eurasian Basin, and analyses of data from 1950–2010 show a decadal warming of this water mass since the late 1970s (Polyakov et al., 2012), as well as a shoaling, by 75 to 90 m (Polyakov et al., 2010). Arctic surface waters have also warmed, at least in the Canada Basin, from 1993 to 2007 (Jackson et al., 2010).